Washing oily solids

ABSTRACT

Oil residues are removed from contaminated solids, in particular from cuttings produced from drilling a borehole using an oil-based drilling fluid, by contacting the solids with a washing fluid which comprises carbon dioxide and cosolvent under conditions at which carbon dioxide is a supercritical fluid and the cosolvent is in solution in the supercritical carbon dioxide and then separating washed solids from the washing fluid; wherein the washing fluid comprises at least 5% by volume of cosolvent which is one or more organic compounds with a Hildebrand solubility parameter in a range from 16 to 21 MPa 1/2 . The washing process may be applied to drilling cuttings produced offshore.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of GB Patent Application Serial No. 1408943.7, filed May 20, 2014, entitled, “WASHING OILY SOLIDS,” which is incorporated by reference herein.

BACKGROUND

When drilling a borehole with an oil-based drilling fluid, there is a need to remove hydrocarbon residues from the cuttings. Cleaning cuttings to have very low residual hydrocarbon on them has the potential to enable disposal of the cuttings with much less constraint than is imposed if the cuttings retain oil contamination. When drilling offshore, regard for the environment and/or legislative regulation may permit cuttings which have been cleaned adequately to be discharged at sea but prohibit such discharge of oil-contaminated cuttings, with a consequence that they may have be brought ashore for disposal.

Several documents have considered using carbon dioxide as a solvent for washing hydrocarbon from cuttings. Society of Petroleum Engineers paper SPE 63126 examined the use of supercritical carbon dioxide for cleaning cuttings obtained using oil based drilling fluid. The authors considered that it would be an effective process. Later paper SPE 106829 also reported the results of experimental work with supercritical carbon dioxide as leading to a positive outcome. Cleaning solids with supercritical carbon dioxide was also the subject of US 2004/0065353.

By contrast, US 2004/0195152 asserted that the use of supercritical carbon dioxide as in SPE 63126 was not economic and proposed the use of liquid carbon dioxide as a cleaning solvent. More recently WO2011/044260 has demonstrated recovery of oil from cuttings using liquid carbon dioxide at temperatures below 20° C., with a temperature of 0° C. being used in some examples.

These prior documents have all been directed to cleaning cuttings from residues of refined oil used to make an oil-based drilling fluid. Some documents report success in recovering the oil with substantially unchanged composition so that it can be reused.

SUMMARY

The present inventors have appreciated that there would be value in providing a process which is able to do more than remove refined oil from cuttings. It would be valuable to clean solid material which has been mixed with hydrocarbon materials which are more difficult than those encountered in refined oil. These hydrocarbon materials may be drilling fluid additives or may be constituents of unrefined, i.e. crude oil. Crude oil, in contrast with refined oil fractions, contains more polar constituents, including asphaltenes which have poor solubility in carbon dioxide. Cuttings mixed with crude oil will come to the surface when drilling through an oil reservoir and the volume of cuttings may be considerable when drilling a borehole which is deviated to extend laterally within the reservoir. The inventors have also recognised that a process for cleaning solids will also have applicability to washing solid residues after extracting oil from a reservoir in which heavy oil is mixed with inorganic particulate solid, typically sand. Such a reservoir is sometimes referred to as an oil sand or tar sand.

According to one aspect of the subject matter disclosed here, a method for removing oil residues from solids comprises contacting the solids with a washing fluid comprising carbon dioxide and co-solvent under conditions at which the carbon dioxide is a supercritical fluid and the co-solvent is in solution in the supercritical carbon dioxide, wherein the cosolvent is one or more compounds with a Hildebrand solubility parameter in a range from 16 to 21 MPa^(1/2).

The Hildebrand solubility parameter (usually designated δ) of a material is defined as the square root of its cohesive energy density, which is equal to its heat of vaporisation divided by molar volume. It is expressed in units of MPa^(1/2) or in units of (cal/cc³)^(1/2)=cal^(1/2) cm^(−3/2).

The Hildebrand solubility parameter of a mixture can readily be calculated from individual values of the solubility parameter and volume fractions by applying the general formula

δ_(diluent)=Σφ_(i)δ_(i)

where φ_(i) and δ_(i) respectively denote the volume fraction and Hildebrand solubility parameter of an individual constitutent.

The co solvent may also have Hansen solubility parameters in the ranges:

δ_(d) 14.5-19.6 MPa^(1/2) δ_(p) 0-8.2 MPa^(1/2) δ_(h) 0-6.8 MPa^(1/2).

Hansen solubility parameters are a set of three parameters with the same units as the Hildebrand parameter but which denote separately a dispersion component (δ_(d)), a polar component (δ_(p)) and a hydrogen bonding component (δ_(h)). These are related to the Hildebrand parameter δ by the formula

δ²=δ_(d) ²+δ_(p) ²+δ_(h) ²

Both Hildebrand and Hansen solubility parameters have been published for many chemical compounds, for example in “The CRC Handbook of Solubility Parameters and Other Cohesion Parameters” by A F M Barton and “Hansen solubility parameters: a user's handbook” by C H Hansen.

The cosolvent may be an aliphatic compound or mixture of aliphatic compounds which each include at least one of a cycloaliphatic ring, an olefinic unsaturation, an ester group, an ether group. The presence of a cycloaliphatic ring, or unsaturation or ester or ether functionality makes such aliphatic compounds more polar than saturated aliphatic hydrocarbons.

In another aspect the present disclosure provides a method for removing oil residues from solids comprises contacting the solids with a washing fluid comprising carbon dioxide and co-solvent under conditions at which the carbon dioxide is a supercritical fluid and the co-solvent is in solution in the supercritical carbon dioxide, wherein the cosolvent is one or more compounds each of which includes at least one of the following:

a cycloaliphatic ring, an olefinic unsaturation, an ester group, an ether group.

It will be appreciated that the washing fluid may comprise supercritical carbon dioxide, a cosolvent which is one or more compounds which contain a cycloaliphatic ring, an olefinic unsaturation, an ester group or an ether group, and also some further compounds without any said cycloaliphatic ring, olefinic unsaturation, ester group or ether group.

The washing fluid may possibly contain specified co-solvent in an amount from 5% to 40% by volume, possibly from 5% to 20% by volume, further compounds (beside carbon dioxide) in an amount from 0% to 20% by volume, and a balance of supercritical carbon dioxide. The amount by volume of any further compounds without any said cycloaliphatic ring, olefinic unsaturation, ester group or ether group may be not more than half the amount by volume of specified co-solvent compounds.

The solubility parameter of supercritical carbon dioxide is dependent on pressure and temperature. Contacting the solids with the washing fluid may be carried out with conditions of temperature and pressure, and amount and composition of co-solvent such that the Hildebrand solubility parameter of the washing fluid is at least 16 MPa^(1/2) and possibly at least 16.5 MPa^(1/2). The Hildebrand solubility parameter of the washing fluid may possibly be not more than 19 or 18 MPa^(1/2).

After contacting solids with the washing fluid, the washed solids and the used washing fluid may be separated while maintaining conditions of temperature and pressure at which carbon dioxide is supercritical. The separated washing fluid is then cooled and/or reduced in pressure so that the carbon dioxide is no longer supercritical. The carbon dioxide is boiled off as gas which is separated from a residue, which may be a liquid residue, containing constituents of the oil removed from the solids and at least some of the cosolvent.

This residue will require disposal, but will be a smaller quantity of material than the solids prior to washing. This residue may be handled in several ways. One possibility is to burn the residue as furnace fuel. Another possibility is to mix it with produced crude oil so that when that oil is refined at an oil refinery, the residual materials from the used washing fluid become part of the refined products.

Another possibility is to keep this residue separate from produced oil, then distil the residue in a distillation plant allowing the cosolvent to be recovered for re-use. Such distillation need not be done at the location where the solids were washed. Indeed it may be desirable to transport the separated residue to a processing facility elsewhere.

The above methods of washing solids may form part of an overall process which includes drilling a borehole within an oil reservoir, while circulating an oil-based drilling fluid (i.e. a drilling fluid with oil as its continuous phase) from the surface to a downhole cutting tool and then back to the surface, and separating the cuttings from the drilling fluid returned to the surface, prior to washing the separated cuttings as specified above. Alternatively, an overall process may include excavating an oil sand (a mixture of hydrocarbon and inorganic solids) and extracting oil therefrom prior to washing the inorganic solids as specified above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 reproduces the phase diagram of carbon dioxide

FIG. 2 schematically shows the system for washing drill cuttings on an offshore rig;

FIG. 3 shows an overall process including operation of the system of FIG. 2; and

FIG. 4 shows a modified system for washing drill cuttings.

DETAILED DESCRIPTION

An oil-based drilling fluid has a continuous phase which is oil, possibly containing some dissolved additives, and may also have some solids suspended in the oil. Such a fluid is normally made with an oil which is a refined hydrocarbon mixture and may be a petroleum fraction separated from crude oil at a refinery.

Additives which are sometimes included in a drilling fluid may be carbonaceous materials: one such is naturally occurring asphalt termed unitaite or uintahite marketed under the registered trademark Gilsonite.

When drilling within a hydrocarbon reservoir, residues of crude oil will be mixed with the cuttings in the drilling fluid returning to the surface. Crude oil contains heavier and/or more polar materials than those encountered in a refined oil and these include asphaltenes which occur in varying, and sometimes quite substantial amounts in crude oils. They are a group of organic materials in which the molecules contain fused aromatic ring systems and include nitrogen, sulphur and/or oxygen heteroatoms. They are accordingly more polar than the other fractions of crude oil (saturates, aromatics and resins).

Thus, cuttings brought to the surface with oil-based drilling fluid may be mixed with added unitaite and/or asphaltenes from crude oil.

A solvent with a low solubility parameter such as n-pentane or n-heptane (Hildebrand solubility parameter are 14.4 and 15.25 MPa^(1/2) respectively) will dissolve saturated aliphatic hydrocarbons but precipitate asphaltic material. (Asphaltenes are customarily defined as that fraction of crude oil which is precipitated by addition of n-pentane or n-heptane but which is soluble in toluene).

The Hildebrand solubility parameter of carbon dioxide is dependent on both temperature and pressure. At temperatures above about 300° K (27° C.) it has a lower Hildebrand solubility parameter than n-heptane and therefore is not a solvent for asphaltene.

As disclosed in prior documents mentioned above, liquid or supercritical carbon dioxide has been proposed as a washing fluid for drilling cuttings carried to the surface in oil-based mud. The carbon dioxide will serve as a solvent for refined hydrocarbon oil. However, if washing with carbon dioxide fails to remove some (e.g. heavier and/or polar) carbonaceous contaminants from cuttings, the cuttings may still be considered as hydrocarbon-contaminated so that possibilities for the disposal of the cuttings remain restricted.

Although supercritical carbon dioxide has a lower Hildebrand solubility parameter than carbon dioxide which is in a liquid state at temperatures below the critical temperature, it will dissolve greater concentrations of certain more polar cosolvents, thus enabling a washing fluid to dissolve polar and asphaltenic materials contaminating cuttings.

As disclosed herein, the co-solvent is a compound or mixture of compounds with Hildebrand solubility parameter in the range from 16.0-21.0 MPa^(1/2). It may have a Hildebrand solubility parameter of at least 16.4 and possibly at least 17.0. Its Hildebrand solubility parameter may possibly be not over 20.5 MPa^(1/2). The co-solvent may have Hansen parameters in the ranges

-   -   δ_(d) 14.5-19.6 MPa^(1/2)     -   δ_(p) 0-8.2 MPa^(1/2), and     -   δ_(h) 0-6.8 MPa^(1/2)

The upper limits for these ranges exclude materials which are very polar and/or include hydroxyl groups and consequently are poor asphaltene solvents.

Compounds used as cosolvent may be aliphatic compounds as mentioned earlier. Possibly these compounds have from 4 to 70 carbon atoms and possibly they contain only carbon, hydrogen and oxygen atoms. If any oxygen atoms are present, these may be present only in ester or ether groups. It is preferred that the number of carbon atoms is at least 6 or 8. The number may well not exceed 60. It is preferred that hydroxyl groups are absent and ketone or aldehyde groups may well also be absent. Included amongst these compounds are some which are available as natural products or as derivatives of natural products and which can provide an economical alternative to petroleum derived aromatic compounds. Such aliphatic compounds may not include any aromatic ring.

The possibilities include two sub-groups of compounds. The first sub-group is hydrocarbons having from 8 to 20 carbon atoms, possibly from 10 to 15 and more especially 10 carbon atoms, including at least one cycloaliphatic ring and/or including at least one olefinic unsaturation. This category includes some naturally occurring terpenes, notably pinenes and limonene. Limonene, which is a C₁₀ terpene, is a natural product available commercially in substantial quantities. These hydrocarbons may well have Hildebrand solubility parameter at the low end of the range, such as from 16.0 to 16.4 MPa^(1/2).

The second sub-group of aliphatic is aliphatic esters incorporating alkyl or alkenyl groups of 6, possibly 10, up to 22 carbon atoms.

This esters category can itself be divided into three parts. The first is esters of formula

R₁—COO—R₂

wherein R₁ and R₂ are straight or branched aliphatic hydrocarbon chains optionally including olefinic unsaturation and having a length of 1 to 22 carbon atoms and where R₁ and R₂ together contain 12 to 44 carbon atoms.

One possibility within this formula is esters of long chain alcohols, so that R₁ is methyl ethyl, propyl or butyl and R₂ has 10 to 22 carbon atoms. Another possibility is esters where both R₁ and R₂ contain at least five carbon atoms, possibly from 5 or 6 to 10 carbon atoms. A third and more readily available possibility is alkyl esters of long chain fatty acids in which R₁ contains 11 to 22 carbon atoms and R₂ is methyl ethyl, propyl or butyl. These can be obtained by transesterification of natural oils and fats. Such esters may be esters of saturated acids, for instance methyl stearate, or esters of unsaturated acids, for instance methyl oleate.

A second part of the esters category is diesters of formula

R₁—Z₁—R₃—Z₂—R₂

where Z₁ and Z₂ are each COO or OCO, R₁, R₂ and R₃ are straight or branched aliphatic hydrocarbon chains optionally including olefinic unsaturation and each having a length of 1 to 22 carbon atoms, and R₁, R₂ and R₃ together contain from 12 to 66 carbon atoms.

A third part of the esters category is triglycerides of formula

R₁COO—CH₂—CH(OCOR₄₎—CH₂—OCOR₂

where R₁, R₂ and R₄ are straight or branched aliphatic hydrocarbon chains optionally including olefinic unsaturation and each having a length of 1 to 22 carbon atoms, and R₁, R₂ and R₄ together contain from 12 to 66 carbon atoms. Triglycerides of acids with 12 or more carbon atoms have similar solubility parameters and it may be both convenient and economical to use a mixture of triglycerides provided by one or more natural oils, in which case R₁, R₂ and R₄ may each have from 11 to 17 carbon atoms. More particularly, palm oil may be used.

Solubility parameters of some examples of these materials are set out in the following table:

Hansen parameters Hildebrand δ_(d) δ_(p) δ_(h) δ Compounds (MPa^(1/2)) (MPa^(1/2)) (MPa^(1/2)) (MPa^(1/2)) d-limonene 16.4 0.2 0.2 16.4 α-pinene (racemic) 15.5 4.3 0 16.1 methyl ester of 16.2 4.9 5.9 17.9 soybean oil cyclohexane 16.8 0 0.2 16.8 glyceryl tripalmitate 18.0 4.3 4.3 19.0

These aliphatic materials have good solubility in supercritical carbon dioxide. Notably, limonene has a solubility in supercritical carbon dioxide which increases very rapidly with increasing pressure, reaching as high as 50% by weight above 100 bar at 45° C. (as reported by Berna et al, J. Chem. Eng. Data 2000 Vol 45 pp 724-7). One possibility here is to use a mixture of materials from both the hydrocarbons and esters subgroups to provide an asphaltene solvent. An example of such a mixture would be limonene and methyl soybean oil. Another would be a mixture of limonene and palm oil. Such mixtures would be more polar than limonene alone, and would also have good solubility in supercritical carbon dioxide.

As shown by the phase diagram which is FIG. 1, carbon dioxide at atmospheric pressure exists as a gas when at all temperatures above 200° K (−73° C.). Carbon dioxide can exist in a liquid state, under pressure, provided temperature and pressure are above the triple point values of 216.4° K (−56.6° C.) and 5.1 bar (518 kPa). If the temperature and pressure are above the critical point values, shown as dotted lines in FIG. 1, at 301.4° K (31.1° C.) and 7.38 MPa, carbon dioxide exists as a supercritical fluid.

FIG. 2 shows schematically a system for washing particulate solids, such as drill cuttings, with a mixture of supercritical carbon dioxide and a co-solvent as the washing fluid. The system includes a loop of connected parts for circulating carbon dioxide. These connected parts include a pressure vessel 10 for washing cuttings, a pressure vessel 12 for separating the carbon dioxide from oil residues and a condenser 14 for returning carbon dioxide from gas to supercritical fluid.

The system has a storage vessel 20 for cuttings waiting to be washed, from which cuttings can be transferred through a valve 22 to the pressure vessel 10 where washing takes place. The vessel has motor 24 operating one or more internal parts 26 for agitating its contents. The vessel 10 also has a surrounding heating jacket 28 for heating to a temperature above the critical temperature of carbon dioxide. A pump 30 is used for charging the vessel with carbon dioxide to a pressure above the critical pressure so that the carbon dioxide becomes a supercritical fluid. A metering pump 32 delivers cosolvent liquid from supply line 34 into the pressure vessel 10.

This system is operated as a batch process. The vessel 10 is charged with cuttings to be washed, carbon dioxide and cosolvent, and the temperature and pressure within the vessel 10 are raised until the carbon dioxide becomes supercritical with the cosolvent in solution in the supercritical carbon dioxide. The contents of the vessel are then agitated for a time to wash the cuttings.

After washing the cuttings, the fluid is displaced from the vessel 10 by operating pump 40 to pump in water. The used washing fluid exits through valve 42 and filter 44 to the second pressure vessel 12. This vessel 12 is then closed off by valve 46 and pressure in the vessel 12 is reduced so that the carbon dioxide in the used washing fluid boils off as gas. Gaseous carbon dioxide leaves through valve 48 and is converted to liquid form by the condenser 14 which passes the liquid carbon dioxide to the pump 30. If required, additional carbon dioxide can be admitted from a supply line 52.

Washed cuttings and water which had been admitted to the vessel 10 are emptied through its bottom outlet valve 54. After carbon dioxide is boiled off in the vessel 12, the liquid residue in that vessel is a mixture of the co-solvent, crude oil from the reservoir and refined oil from the drilling fluid. This mixture is discharged from the vessel 12 through its bottom outlet valve 56.

FIG. 3 diagrammatically places the system of FIG. 2 in the context of an overall procedure. Drilling, using an oil-based fluid, takes place on an offshore rig. The drilling fluid which returns to the surface is separated from drilling cuttings by conventional separation and is re-used. This initial separation is carried out on the drilling rig. The cuttings which have been separated are still mixed with some oil. These oily cuttings are then washed using the system shown in FIG. 2. The apparatus of FIG. 2 may be located on the rig, or on a support vessel in the vicinity of the rig. The washed cuttings are discharged into the sea. The liquid residue from the separation pressure vessel 12 is transported to an on-shore processing facility indicated as and distilled to separate it into light oil which is re-used for making oil-based drilling fluid, co-solvent which is re-used in the washing process, and a final residue which is asphaltic.

As a possible alternative, the residual liquid from the separation pressure vessel 12 may be added to oil refinery feedstock so that its constituent parts become a small part of the refinery products.

FIG. 4 shows a variation of the system of FIG. 2. When the used washing fluid is discharged from the pressure vessel 10, it goes to a first separation vessel 60 where it is cooled below the critical temperature and also reduced in pressure. The carbon dioxide becomes a liquid with hydrocarbons from the drilling fluid dissolved in it. At least part of the co-solvent separates as a second liquid which has most of the asphaltene dissolved in it.

These two liquid phases are separated at the vessel 60. The liquid carbon dioxide solution is transferred to another separation vessel 62. The second liquid from vessel 60 is pre-dominantly cosolvent containing dissolved asphaltenes. This is de-pressurised in vessel 64 so that any carbon dioxide boils off and is sent to the condenser 14 along line 66. The liquid is then discharged through outlet 68 and sent for processing on-shore, as in FIG. 3.

The liquid carbon dioxide solution transferred into separation vessel 62 is reduced in pressure allowing the carbon dioxide to boil off as gas which goes along line 70 to the condenser 14 where it is condensed to liquid carbon dioxide. The liquid remaining in the vessel 62 is oil from the drilling fluid, possibly mixed with some-co-solvent, and this is discharged through outlet 72 and re-used in drilling fluid.

The systems shown in FIGS. 2 and 4 operate as batch processes in which each vessel must be emptied before the steps performed at that vessel can be repeated. To increase throughput, a more complex system could have duplicate pressure vessels 10 operating alternately so that one of the vessels is being filled, while the other is being emptied. Similarly, there could be duplicate separation vessels 12, and the vessels 60, 62 and 66 of FIG. 4 could all be provided in duplicate, to be operated alternately. It is also possible to construct apparatus to operate as an even more continuous process with incoming and outgoing materials entering and leaving through pressure locks.

Heavy oil from an oil sand may be extracted by the CHOPS (Cold heavy oil production with sand) process which produces sand to the surface with oil and the separates the sand at the surface. The separated sand is contaminated with oil.

This contaminated sand may be washed using the system of FIG. 2. The cosolvent may be an ester or an aromatic hydrocarbon fraction from a refinery. The residual liquid from the separation vessel 12 may be added to the produced heavy oil. 

1. A method for removing oil residues from solids contaminated therewith comprising contacting the solids with a washing fluid which comprises carbon dioxide and cosolvent under conditions at which carbon dioxide is a supercritical fluid and the cosolvent is in solution in the supercritical carbon dioxide and separating washed solids from the washing fluid; wherein the washing fluid comprises at least 5% by volume of co-solvent which is one or more organic compounds with a Hildebrand solubility parameter in a range from 16 to 21 MPa^(1/2).
 2. A method for removing oil residues from solids contaminated therewith comprising contacting the solids with a washing fluid which comprises carbon dioxide and cosolvent under conditions at which carbon dioxide is a supercritical fluid and the cosolvent is in solution in the supercritical carbon dioxide and separating washed solids from the washing fluid; wherein the co-solvent comprises at least 5% by volume of co-solvent which is one or more aliphatic organic compounds, each of which includes at least one of the following: a cycloaliphatic ring an olefinic unsaturation an ester group an ether group.
 3. A method according to claim 1 further comprising separating washed solids from used washing fluid, subsequently altering the temperature and/or pressure of the separated used washing fluid so that the carbon dioxide therein becomes gaseous and then separating the gaseous carbon dioxide from liquid or solid residue.
 4. A method according to claim 1 preceded by drilling a borehole within an oil reservoir, while circulating an oil-based drilling fluid from the surface to a downhole cutting tool and then back to the surface, and separating the cuttings from the drilling fluid returned to the surface, prior to washing the separated cuttings.
 5. A method according to claim 1 preceded by excavating an oil sand and extracting oil therefrom prior to washing the inorganic solids.
 6. A method according to claim 1 wherein the co-solvent consists of carbon, hydrogen and oxygen atoms only.
 7. A method according to claim 1 wherein the co-solvent comprises at least one aliphatic compound containing 5 to 20 carbon atoms and having at least one cycloaliphatic ring and/or at least one olefinic unsaturation.
 8. A method according to claim 1 wherein the co-solvent comprises at least one aliphatic ester containing at least one a saturated or unsaturated aliphatic chain of 6 to 22 carbon atoms.
 9. A method according to claim 8 wherein the said co-solvent comprises at least one ester of the general formula R₁—COO—R₂ where R₁ and R₂ are straight or branched aliphatic hydrocarbon chains optionally including olefinic unsaturation and having a length of 1 to 22 carbon atoms and where R₁ and R₂ together contain 12 to 44 carbon atoms.
 10. A method according to claim 9 wherein R₁ comprises a straight or branched aliphatic hydrocarbon chain optionally including olefinic unsaturation and having a length of 11 to 18 carbon atoms and where R₂ comprises methyl, ethyl, propyl and/or butyl.
 11. A method according to claim 8 wherein the said co-solvent comprises diesters of formula R₁—Z₁—R₃—Z₂—R₂ where Z₁ and Z₂ each represent COO or OCO, R₁, R₂ and R₃ are straight or branched aliphatic hydrocarbon chains optionally including olefinic unsaturation and each having a length of 1 to 22 carbon atoms, and R₁, R₂ and R₃ together contain from 12 to 66 carbon atoms.
 12. A method according to claim 8 wherein the said co-solvent comprises triglycerides of formula R₁COO—CH₂—CH(OCOR₄₎—CH₂—OCOR₂ where R₁, R₂ and R₄ are straight or branched aliphatic hydrocarbon chains optionally including olefinic unsaturation and each having a length of 1 to 22 carbon atoms, and R₁, R₂ and R₄ together contain from 12 to 66 carbon atoms.
 13. A method according to claim 12 wherein R₁, R₂ and R₄ each contain from 11 to 21 carbon atoms.
 14. A method according to claim 1 wherein the co solvent has Hansen solubility parameters in the ranges: δ_(d) 14.5-19.6 MPa^(1/2) δ_(p) 0-8.2 MPa^(1/2) δ_(h) 0-6.8 MPa^(1/2).
 15. A method according to claim 1 wherein contacting the solids with the washing fluid is carried out with conditions of temperature and pressure, and with an amount and composition of co-solvent such that the Hildebrand solubility parameter of the washing fluid is at least 16 MPa^(1/2) 